University Electrical Engineering: 15 Hard Questions

Assessment context

What is active, reactive, apparent & complex power? Power Factor is a measure of how effectively incoming power is used in your electrical system I) Active power: It is the actual power that is delivered to the load II) Reactive Power: The powers that continuously bounce back and forth between source and load is known as reactive power. III) Apparent Power: It is the product of voltage & current without referencing the phase difference between them. III) Complex power: it is the product of voltage & current with reference to the phase difference between them. it is the complex sum or vector sum of the active power & reactive power. It is denoted by S & measure in VAR. 5.What is a leading & lagging power factor & how can you improve it? State the methods of power factor correction The cosine of the angle between voltage & current is called the power factor. If the current leads the voltage, it causes a leading power factor. If the current lags voltage the power factor is lagging. Power Factor Correction is a technique which uses capacitors to reduce the reactive power component of an AC circuit in order to improve its efficiency while real power do not change. Most loads are inductive that causes a lagging power factor. Thus, these following methods are used for p.f correction (to nullify the lagging current): - Static Capacitor The capacitor helps in providing a leading current that eliminates the lagging component of current & improves the power factor Synchronous Condenser It is an over-excited synchronous motor with no load that also provides a leading power factor. Phase Advancer Phase advancer is a simple AC exciter which is connected on the main shaft of the motor and operates with the motor’s rotor circuit for power factor improvement. Phase advancer is used to improve the power factor of induction motor in industries 7.Why do we improve the power factor? Large Line Losses (Copper Losses): Line losses (I2R) depend on current. The low power factor draws a large amount of current as compared to the high-power factor. Large kVA rating Equipment: PF is inversely proportional to KVA. Low PF equipment with a high KVA rating is larger in size. Large Conductor Size and Cost: we need large conductors to transmit the heavy current required due to low power factor. Poor Voltage Regulation and Large Voltage Drop: The large current due to low PF causes a high voltage drop that needs to be regulated more often than usual. Low Efficiency: The losses due to the high current flow & voltage drop deteriorate the efficiency of the system. The efficiency is maximum at PF=1. How do you calculate capacitor value for PF? Why reactive power is important? Reactive power is unavoidable in power supplies. This is so because majority of load is motors, electromagnets, transformers all of which use magnetic fields for their operation. All these applications result in currents which lag behind voltage in phase. The current for creating the magnetic field just do this- create the field, and do not take part in any active power or in getting work done. This current is reactive by definition, and no machine can work without this reactive component. What is a linear & non-linear circuit? In a linear circuit, the relation between the current & voltage is linear i.e. resistance, inductance While in the non-linear circuit, the current & voltage does not have a linear relationship i.e. diode What could be the reason for the current to double in a linear circuit? There are two reasons for increasing the current: Either the total resistance of the circuit is reduced by half Or the supply voltage to the circuit is doubled. What is a primary & secondary cell? The primary cell is a non-rechargeable battery. Use toys, baby car, remote The secondary cell is a rechargeable battery. Use cell phones, vehicles, generators, etc. What are the limitations of ohm’s law? Ohm law is not applicable to non-linear circuit. The criterion for ohm’s law is that the resistance must be constant which also depends on the temperature. Diode voltage vs curve does not applicable to ohm law Does current lead or lag the voltage in an inductive or capacitive circuit? The current lags the voltage in an inductive circuit. the current leads the voltage in a capacitive circuit. Define the term Capacitance and Inductance Capacitance: capacitance is the ability of a component to store charge between two plates when there is a potential difference applied. Inductance: inductance is the ability of a conductor to store charge by varying magnetic field in conductor coil. Why the Capacitors works on AC only? In ac there is frequency What is the maximum power transfer theorem? the maximum power will be transferred from source to the load when the external load resistance equals the internal resistance of the source or Thevenin’s resistance of the circuit. Explain Thevenin’s Theorem in a single sentence. one voltage and resistance connected in series. Explain Norton’s Theorem in a single sentence. to an equivalent circuit with just a single current source and parallel resistance connected to a load. What are the different colors on wires indicates? Or Mention what are the different colors on wires indicates? The different colors of the wires are used for phase indication purposes. Single phase: Red and black 3 phases: Red, Yellow, Blue Neutral: white and gray Ground: green or green with yellow strip What is the difference between a single pole and a double pole circuit breaker? Single-pole breakers are wired with one hot wire and one neutral wire. When there is an overload in a single-pole breaker’s circuit, only that particular breaker trips. Double-pole breakers have two hot wires that are connected by a single neutral wire. That means if there’s a short circuit on either of the pole’s hot wires, both trips. Why Motor rated in kW instead of kVA? As we know the transformer is rated in KVA because its PF depends on the nature of the loads. But transformer is not a load. However, Motor is a load and has a fixed Power factor, the rating has been mentioned in kW What is the definition of generator & motor? The working principle of a motor is based on the current-carrying conductor that experiences a force when it is kept in the magnetic field. An electric generator is a machine that converts mechanical energy to electrical energy. The working principle of generator is based on electromagnetic induction. What is a motor starter? Motor Starter is a device that connects in series with the motor to decrease the starting current. The purpose of a starter is not to just start the motor, but it performs the two main functions. They are as follows: • To reduce the heavy starting current, • To provide overload and under-voltage protection.